The present invention relates to a backlight for illuminating a display section including a liquid crystal display element and an operation section including button switches. More particularly, the invention relates to a backlight device and a backlighting element, which are constructed with an EL panel, LEDs, lamps and others.
There is known a device equipped with a liquid crystal display panel (referred to as an LCD panel) having a liquid crystal display element (LCD) and an operation panel including panel switches associated with operation buttons. Recently, this type of device has been equipped with a backlighting function to illuminate the reverse side of the display section and the operation section in order to render display characters and characters on switches to be legible even in a dark place. In this type of communication device, those characters must be legible in every use condition. Therefore, the backlighting function is indispensable in particular for mobile communication devices, such as portable phones and pagers.
An electrical arrangement of a device with a conventional backlight device is shown in FIG. 12. As shown, the device is made up of a display unit 101, a backlighting element 102, a backlight driver 103, a control unit 104, a switch 105, and a power supply 106. The display unit 101 includes a LCD for displaying characters, for example. The backlighting element 102 includes an EL (electroluminescence) panel for illuminating the back of the display unit 101. The backlight driver 103 drives and controls the backlighting element 102. The control unit 104 controls the display unit 101 and the backlight driver 103. The switch 105 is provided for backlighting. The power supply 106 is used for supplying electric power to the whole device.
FIGS. 13 and 14 cooperate to show a structural arrangement including a display unit and a backlighting element. FIG. 13 shows a side view of a structure including the display unit and the backlighting element. FIG. 14 shows a plan view of the backlighting element. An LCD panel 101a forms a display unit 101. An EL panel 102a forms a backlighting element 102. The EL panel 102a is disposed on the rear side of the LCD panel 101a. Light emitted from the EL panel 102a passes through the LCD panel 101a, and is irradiated from the front broad side thereof. Thus, a structure in which a light is irradiated from the back of the LCD panel 101a is employed. A light emitting section 107 is formed in the EL panel 102a, while covering the entire viewing area of the LCD panel 101a. 
Characters and/or images are displayed in the display unit 101 under control by the control unit 104. In this case, the backlighting element 102 is lit in order to secure a satisfactory viewing in a dark place. To light the backlighting element 102, the switch 105 is operated. In response to this, the control unit 104 sends an operation instruction to the backlight driver 103. In turn the backlight driver 103 applies a drive signal having an AC waveform (e.g., a sinusoidal waveform, a triangle waveform, or a rectangular waveform) at the amplitude of, for example, 50 to 100 Vpp, to the backlighting element 102. Then, light emitting elements within the backlighting element 102 is excited to emit light and illuminate the display unit 101. Electric power consumed by the backlight driver 103 and the backlighting element 102, when those operate, depends on the area of and luminance brightness on the light emitting section of the backlighting element 102. The power consumed is generally 10 to 50 mA for the size of approximately 60 mm×30 mm. After the backlighting element is lit, the backlighting element 102 automatically lights of f under control by the control unit 104 when the following condition is satisfied; viz., the user intentionally lights off the backlighting element or the device is not operated for a predetermined time (several tens seconds to several minutes).
In the conventional backlight device thus constructed, power consumption is large when the backlighting element is lit. When it is applied to the mobile communication devices designed placing emphasis on the portability, such as pagers, portable telephones, PHS terminals, and PDA terminals, reduction of the communication device entails a restriction on the power capacity of the power supply (e.g., a battery). Therefore, the lighting of the backlighting element will greatly influence the battery lifetime. In case where a battery used for such a communication device as an air zinc battery is small in capacity, when the backlighting element is lit to consume large power, the battery voltage will drop. A conventional measure, generally taken for this, is to reduce the power consumption by reducing the luminance brightness of the backlighting element.
Where the EL panel is used, a signal of an AC waveform, e.g., a triangle waveform, needs to be generated for a drive signal during the lighting of the backlighting element, and its power is large. Because of this, the drive signal will cause power source voltage to vary and produce unwanted spurious radiations. Those voltage variation spurious radiations will cause noise and deteriorate radio and acoustic performances of the backlight device. For this reason, additional components are required for noise shielding. Device complication and production cost increase result.
The enhancement of the device function increases the areas that are required for the display section and the operation section of the backlight device, and increases the backlight emitting area. With this, more electric power is consumed for securing a required backlight brightness. Therefore, the power consumption problem is more serious.